If someone asked me what I wanted to do in five years, I would say, "I have no clue." That’s because I’ve switched roles every year for the past three years. My current boss probably has the 10-month mark on her calendar as a time to start getting nervous.

But if you asked me what I wanted to do when I grew up? I would say, "I want to have made a difference. And I don’t have to wait to grow up to do that."

Legal limits on greenhouse-gas emissions are coming fast, with a $1 trillion carbon market emerging. At the core: A cadre of young, idealistic Yale forestry grads. But will carbon offsets do anything to slow global warming?

Most of Vinod Khosla's environmental startups are efforts to reduce our dependence on petroleum — both by making better use of oil and by finding other sources of liquid fuel. The rest address solar and geothermal power and desalination of seawater. Very broadly, his green-tech portfolio falls into three clusters: